Fix media attachment size validation not correctly accounting for file type (#16819)

Fix media attachment size validation not correctly accounting for file type (#16819)

* Fix media attachment size validation not correctly accounting for file type

Fixes a regression introduced in #16724 caused by the fact that kt-paperclip
now correctly runs validations before processing, meaning that file size
verification could not rely on our before_post_processing hook.

Moved the `before_post_processing` hooks to `before_validate` to make sure
the media attachment type is set correctly before the file gets validated.

* Add tests